False positive `[operator]` on union of dicts with heterogeneous value types

Bug Report

If I take the union of two dicts with different value types, I cannot assign the result to a variable that is annotated with the resulting type. Doing so causes an error. Assigning to an non-annotated variable does not cause an error, which seems like the correct behavior.

Do I really need the annotation on d5 below? No. But I have a non-toy example of this in a code base where d5 is a type annotated attribute on an object, which results in the same error.

d1: dict[str, str] = {}
d2: dict[str, str | None] = {}

d3 = d1 | d2
reveal_type(d3)  # builtins.dict[builtins.str, Union[builtins.str, None]], aka dict[str, str | None]

# This is fine
d4: dict[str, str | None] = d3  # aka d1 | d2

# This isn't?
d5: dict[str, str | None] = d1 | d2

To Reproduce

See: https://mypy-play.net/?mypy=master&python=3.13&flags=show-error-codes&gist=2643cb3ff1ea956a1c4ed0869140d46d

Expected Behavior

I expect d5: dict[str, str | None] = d1 | d2 to be legal.

Actual Behavior

error: Unsupported operand types for | ("dict[str, str]" and "dict[str, str | None]") [operator].

Your Environment

• Mypy version used: 1.13.0
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12

Inizia con la riproduzione minima nell’issue o nell’esempio mypy-play collegato, usando il contesto indicato di Python 3.12 e mypy 1.13.0. Confronta l’unione diretta annotata con il caso della variabile intermedia, quindi verifica che l’assegnazione diretta non produca più l’errore [operator], mentre il tipo rivelato rimanga dict[str, str | None].